An incinerator produces a hot waste gas that contains sulfur compounds as well as particles that must be removed before the gas can be released to the atmosphere. The sulfur content of the waste gases is reduced with relatively high efficiency by the wet method wherein water mixed with lime or calcium-oxide additives is sprayed through the sulfur-containing gas stream, absorbing and neutralizing the sulfates. This essential process substantially lowers the temperature of the waste gas. The particles can be stripped from the gas by a cyclone, electrostatic precipitator, or other filter arrangement.
Before gas cooled by the wet desulfurizing process can be released to the atmosphere it is necessary to reheat it, thereby lowering its relative humidity. Otherwise the liquid in the waste gas will condense out in the stack or immediately on leaving same, creating a pollution problem on the outside the stack and a corrosion problem inside it.
It was standard practice up to the last few years, as described in Swiss Pat. No. 376,475, to divert a small portion of the very hot particle- and sulfur-containing waste gas from the regenerative preheater and inject it into the stream of waste gas downstream of the desulfurizing scrubber, when it is relatively cool. This style of reheating uses no fuel, however the sulfur content of the waste gas is increased by this addition, so that overall the separation efficiency is reduced.
Scrubbed waste gas can also be reheated in heat exchangers when heated gas is injected between the heat exchanger and scrubber to lower the relative humidity. This dries impurities which are easily separated out. The additive hot gas is thus taken up in the waste gas. The reheating of the scrubbed waste gas is effected with the hot and dirty waste gas in a heat exchanger.
In another method for heating cleaned, that is desulfurized and filtered, incinerator waste gas which is to be released into the atmosphere, the waste gas is cleaned by direct contact with a scrubbing solution at a temperature of from 20.degree. C. to 65.degree. C. and is saturated with water vapor. Then the mixture is heated in a second heating stage by indirect heat exchange with a heat-exchange medium to a temperature of 110.degree. C. to 140.degree. C. and released into the atmosphere. The hot gas necessary for the first heating stage is warmed in a heat exchanger, e.g. by means of steam. The difficult part of heating the clean waste gas is in the second heating stage, so that here either the dirty waste gas or water must be used.
With both of these processes described immediately above the cleaned waste or stack gas is reheated in a heat exchanger, thereby reducing the overall efficiency of the process.
It is further known to use a portion of the air preheated for the incineration to improve the efficiency of a blower having hollow blades. The temperature of the air fed to the blades is about 200.degree. C. to 300.degree. C. so that a condensing of the steam in the waste gas at about 140.degree. C. and the resultant corrosion of the blades and housing of the blower are avoided, as compared to how they develop when cold air is used. The injected air does not, as a result of the small volume and small temperature differential, serve to warm the cool waste gas, but instead only serves as is known to improve the performance of the waste-gas blower.
In recent times, as described in German patent document No. 2,724,030, a gas preheater has been developed which draws further heat from the waste gas after the regenerative preheater and before the scrubber and uses this heat according to regenerative principles to reheat the waste gas. This waste gas reheater is made corrosion-resistant, but has the operational disadvantage that entrained gypsum particles can build up on the heat-exchanger surfaces. Overcoming this problem has been attempted by selection of materials and cleaning procedures. A further disadvantage of the gas preheater is that dirty gas can leak through loose joints into the cleaned gas, thereby reducing the efficiency of the scrubber.
A further process is described in German patent document No. 3,110,361. Here the heat exchanger is fed a large volume of air, one that is more than is necessary to support the combustion. The excess air is added to the cool desulfurized waste gas before it enters the stack so as to reheat it. Such an arrangement lowers the temperature of the incinerator combustion air, even when a separate steam preheater is employed, so that extra fuel must be used to support the combustion.
Other systems are described in British Pat. No. 2,090,959 of Trojani, French Pat. No. 1,315,440, and U.S. Pat. Nos. 3,320,906 of Domahidy, 3,530,806 of Bowman, 3,986,848 of Howell, 4,149,453 of Reed, and 4,245,569 of Fallon.